Metal powders having fine, spherical particles are utilized in the metallurgical, chemical, and electronics industries in the manufacture of a wide variety of products. For applications of metal powders in the electronics industry, it is desirable to produce such powders at high yield and reasonable cost for the preparation of solder pastes for use in the fabrication of high-density electronic circuit boards. In addition, it is desirable that the powders be uniform and free-flowing. An extensive review of methods for making free-flowing metal powders is given by S. J. Savage and F. H. Froes in an article entitled "Production of Rapidly Solidified Metals and Alloys" in Journal of Metals, April 1984, pp. 20-32. One category of processes involves the mechanical dispersion of a stream of molten metal into fine droplets followed by quenching and cooling to form solidified particles. Dispersion of the molten metal into droplets is accomplished by (1) impinging one or more high-pressure gas or liquid streams onto the stream of molten metal, (2) dispersing the molten metal stream by various centrifugal means such as rotating discs, cups, or drums, or (3) forming droplets by passing the stream of molten metal between two counter-rotating rollers. Cooling of the molten droplets formed by these methods is accomplished by contact with cool gas, often by the droplets freely falling through a gas-filled cooling tower, or by contacting the droplets with a cooling liquid such as water, oil, or a cryogenic liquid. In two related methods, molten droplets are formed by a rotating consumable arc electrode which emits the droplets tangentially into a cooling gas for solidification, or by directing molten metal from a consumable electrode by electric arc or electron beam. Droplets are cooled by contacting with a high-velocity cooling gas such as helium. All of the methods reviewed in this article are characterized by (1) utilization of mechanical means to form molten metal droplets from previously melted metal and/or (2) the disintegration of a stream of previously melted metal using compressed fluids to form the droplets.
In a method related to those discussed above, a stream of molten metal is dispersed by the impingement of high-velocity streams of a cryogenic liquid followed by further cooling in a bath of the same liquid. Powder product is then recovered from the cryogenic liquid in one or more fractions. Such methods are disclosed in U.S. Pat. Nos. 3,646,177 and 4,897,111, and in an article by H. W. Bergmann et al entitled "Production of Metallic Powders by Atomization of Metallic Melts With Liquid Gases" in Fertigungs-Technologie, Volume 65, No. 6, 1988, pp. 513-531. The use of powders made by this method in solder pastes for the fabrication of electronic devices is discussed in a paper entitled "Solder Paste Requirements for Fine-Pitch Technology" by J. P. Langan in Circuits Assembly, October 1990, pp. 51-54. The importance of low oxide content and spherical particles in these powders is pointed out in this paper. An analysis of the gas atomization method discussed above is given by R. A. Ricks and T. W. Clyne in an article entitled "Bulk Production of Ultrafine Metallic Powder by High Pressure Gas Atomization" published in J. Mat. Sci. Letters 4, (1985), pp. 814-817. The authors studied the effects of nozzle design on particle size distribution in tin and tin-lead powders.
M. G. Chu et al describe an alternate method of making small, highly-undercooled lead-tin alloy particles in an article entitled "Solidification of Highly Undercooled Sn-Pb Alloy Droplets" in Metallurgical Transactions A, Vol. 15A, July 1984, pp. 1303-1310. The method is useful for studying the formation and morphology of such droplets and comprises making an emulsion of molten alloy droplets in a high-boiling organic liquid and injecting the emulsion into cold carbon tetrachloride to solidify the droplets. This method is not appropriate for use in commercial production.
An alternate approach to droplet and particle formation differing from the methods described above involves the use of plasma arc systems to form molten droplets which are directly quenched and solidified by various methods. U.S. Pat. No. 3,041,672 discloses a method of feeding a consumable rod or wire into the discharge of a plasma arc wherein the rod is melted and dispersed into fine droplets by the discharged plasma arc gas. The droplets and plasma gas are passed into a collector which cools the particles to form a powder product; the collector contains a cooling liquid such as water. Steel, tungsten, and sapphire particles can be made with average particle diameters between about 150 and 300 microns.
U.S. Pat. Nos. 4,592,781, 4,613,371, and 4,687,501, and European Patent Application EP 0 134 808 B1 disclose a method for making ultrafine metal powders which comprises feeding a powder through an arc or induction plasma torch to generate a stream of molten metal droplets, directing the droplets onto a repellent surface, and solidifying the particles rebounding from the surface. Powders can be made in which at least about 80% of the particles have diameters less than 10 microns; however, the repellant surface quenching mode results in predominantly non-spherical particle shapes.
A process for producing fine particles of metal or alloy is disclosed in U.S. Pat. No. 4,376,740 wherein a molten metal or alloy is atomized by a stream of argon and/or helium heated in a high-temperature plasma, and the molten particles are cooled and recovered as a powder in a cold trap. Particles having diameters less than 5 microns can be made by this process. U.S. Pat. No. 4,732,369 describes an apparatus which operates in a similar manner using a plasma arc to atomize a molten metal or ceramic material, wherein the atomized material is removed by vacuum or suction into a cold trap for solidification and collection as a powder product. Solidification can be promoted by blowing a jet of cold gas on the stream of atomized material as it enters the cold trap.
An article by K. Ishizaki et al entitled "Direct Production of Ultrafine Nitrides and Carbides Powders by the Plasma Arc Method" in the Journal of Materials Science 24 (1989), pp. 3553-3559 discusses a method for producing ultrafine powders using an argon-nitrogen plasma gas in conjunction with ammonia-methane mixtures injected into a plasma reaction chamber. Tungsten and titanium carbide powders can be produced with particle diameters in the 5-10 nanometer range; aluminum powders can be made in the 20-50 nanometer range. Low powder yields and explosive/corrosive plasma gases make this method unsuitable for commercial powder production.
U.S. Pat. No. 4,781,754 discloses a method for making fine alloy powders by introducing the alloy as particles into a plasma torch and directing the torch discharge onto an inner surface of a rotating thermally conductive quench cylinder whereby the molten alloy particles are rapidly solidified to yield a powder product. The inner surface of the quench cylinder is typically cooled by applying a liquefied gas such as argon as the cylinder rotates. This method does not produce highly spherical powders. U.S. Pat. No. 4,952,144 describes an apparatus in which molten particles from a plasma torch are quenched, cooled, and solidified by passing the plasma torch discharge through a spray of cryogenic liquid to form a powder product which is captured in a cyclone collector.
Japanese Patent Application No. 2-290245 discloses a method for making uniform ceramic or metal powders of high fluidity by feeding a powder through a plasma torch to form molten droplets which are quenched and solidified in a water bath.
The plasma-based methods described above have certain drawbacks for the large-scale production of metal powders, especially when the droplets must be rapidly quenched. Because the plasma flame is at a very high temperature (typically above 3000.degree. C.), the quench rate of the droplets will necessarily be lower than for alternate methods discussed above due to the large volume of hot plasma gas which must be cooled along with the droplets. Because of a short residence time in the plasma flame, injected powders are frequently overheated at the surface and cooler in the core which can lead to metal evaporation and/or nonuniform particles. Further, when a cryogenic liquid is used for quenching, the consumption of the liquid will be high due to the large volume of hot plasma gas. In addition, most plasma methods require a powder feed which is easily contaminated and have generally low throughputs. The methods described above involving solidification of previously molten metal by mechanical or compressed fluid dispersion do not suffer from the drawbacks of plasma-based methods. However, the methods generally have other drawbacks for producing uniform, fine, oxide-free powders such as lack of flexibility for producing small batches of specialized powders, the presence of contaminants in the powder product, the need for complex and expensive equipment, the need to keep the metal in the molten state during dispersion of a stream thereof, and the handling of cryogenic liquids at extremely high pressures.
Given the present state of the art as reviewed above, there is no optimum method for making fine, spherical, free-flowing, oxide-free metal powders utilizing simple, flexible, and economical equipment. There is a need for an improved method to make such powders for use in a broad range of industries. In particular, there is a need in the electronics industry for improved solder pastes which will require powders containing smaller metal particles with higher sphericity and lower oxide content than are currently available. In addition, improved melting properties of the particles in these solder pastes are needed, and such properties can be obtained if higher quench rates can be achieved in the powder production step. The invention described in the disclosure and claims which follow provides a new and improved method for making metal powders having such properties.